Speech quality improving system utilizing the generation of higher harmonic components

ABSTRACT

A speech quality improving system for a band-limited voice signal, comprising a branching circuit for dividing the bandlimited voice signal into two branched signals each having the same waveform as the band-limited voice signal, a higher harmonic signal generator for generating higher harmonic components of one of the two branched signals, and a combining circuit for combining the other of the two branched signals with the generated higher harmonic components to provide a combined voice signal having an improved speech quality realized by increasing the higher harmonic components of the band-limited voice signal. The higher harmonic generator comprises a cascade combination of an instantaneous compressor and a level range expander having reciprocal power characteristics of the compression ratio of the instantaneous compressor.

United States Patent [191 Ishigami et al.

[ SPEECH QUALITY IMPROVING SYSTEM UTILIZING THE GENERATION OF HIGHER HARMONIC COMPONENTS [75] Inventors: Hikoichi Ishigami; Seishi Kitayama;

Akira Sato, all of Tokyo-To, Japan [73] Assignee: Kokusai Denshin Denwa Kabushiki Kaisha, Tokyo-To, Japan 22 Filed: Sept. 21, 1972 21 Appl. No.: 290,898

[30] Foreign Application Priority Data 1 Aug. 6, 1974 3,127,476 3/1964 David 179/1 SA Primary Examinerl(athleen H. Claffy Assistant Examiner.lon Bradford Leaheey Attorney, Agent, or Firm-Robert E. Burns; Emmanuel J. Lobato; Bruce L. Adams [5 7] ABSTRACT A speech quality improving system for a band-limited voice signal, comprising a branching circuit for dividing the band-limited voice signal into two branched signals each having the same waveform as the bandlimited voice signal, a higher harmonic signal generator for generating higher harmonic components of one of the two branched signals, and a combining circuit for combining the other of the two branched signals with the generated higher harmonic components to provide a combined voice signal having an improved speech quality realized by increasing the higher harmonic components of the band-limited voice signal. The higher harmonic generator comprises a cascade combination of an instantaneous compressor and a level range expander having reciprocal power characteristics of the compression ratio of the instantaneous.

compressor.

8 Claims, 13 Drawing Figures AMPLIFIER GENERATOR PATENTEUMIB W 3.828.133

SHEET 1 F 5 AMPLIFIER ,4 a

COMBINER HYBRID BAND PASS OUT PUT (dB) I a I 9 Fig 4 0 I PATENTED 3.828.133

SHEET 2 [1F 5 IF DET- AMLIFIER FILTER EcTOR HYBRID b 7 2 7 4 7 INSTAN: H. ER

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SHEU 3 [IF 5 a ax AMPLI-' b b0 PIER 38 L 4? 37 23 SIDE- L W TONE PAOSS E E HYB I/ CIRCUIT FILTER [RELAYT 34 L 70 31,7 L I HYB d HYB STROKE BAND STROKE SIGNAL PASS SIGNAL TRANS/REC. FILTER DET. 44 I AMPLI- AMPLI- ER PIER 2 HYB I 7 I 35 TQWP w??? COMPRE- SSoR T I EXPANDER AMPLIFIER 9 I I 6 3 a BAND PASS FILTER HYB o Fig. 7

PATENTEUMIE SHEET 5 [1F 5 I02 7 3 2 9 N TAN-I I FREQ, I 707 S PHASE UENCY p /gs COMPRE- 'MODULATOR coNvER- FILTER ssoR TER up T 9 70a PHASE LLA sHIFTER FREQUENCY LOW'PASS ig 7 CONVERTER" FILTER AMPLIFIER 20 4-7 W HIG WEIGHTINIG 202 FILTER'I g QIP cIRcuIT-I 209 207 270 HYB HYB +0 203 204-2 205-2 2062 I LBRA HIGHE? I I 'J NEE FILTER-I1 V HARMONIC F EST F Cm GEN-II ER 207 HIGHER WEIGHTING FILTER III HASQSFJIAC CIRCUIT-111 Fly. 73

SPEECH QUALITY IMPROVING SYSTEM UTILIZING THE GENERATION OF HIGHER HARMONIC COMPONENTS This invention relates to a communications system for a band-limited voice signal.

Telephony is now used by short wave communication circuits, submarine cable and communication satellites. In the short wave communication circuits speech quality is frequently destroyed due to radio interference. Frequency band limitation is desirable for the above communication circuits for reducing the radio interference and for improving the maximum quantity of transmissible information in a limited frequency band.

Many band-limiting systems have been heretofore proposed for such purposes. In one (e.g., Japanese Pat. Publication No. 373965/1962) of the proposed systems, frequency components of (0.3 to 1.5) kilo-Herz are derived from an original voice signal of (0.3 to 3.4) kilo-I-Ierz at a point of origin and transmitted to a receiver-where an improved voice signal like the original voice signal is obtained by combining the transmitted voice signal of (0.3 to 1.5) kilo-Herz with high frequency components, which are derived from the transmitted voice signal so as to approximate the frequency components of the original voice signal attenuated at the point of origin. However, while harmonic relationship among the fundamental wave and harmonic waves can be maintained since sideband components of frequency components of the transmitted voice signal are used, levels of the sideband components are not proportional to those of the transmitted voice signal and natural quality of the reproduced signal is insufficient since an ordinary nonlinear circuit is employed for generating the sideband components.

An object of this invention is to provide a highquality signal processing system for a band-limited voice signal capable of providing high speech quality not less than the speech quality of an ordinary tele phone circuit.

In accordance with the principle of this invention, high frequency components each having a level proportional to a band-limited voice signal are developed from the band-limited voice signal by the use of a combination of an instantaneous compressor and an expander and are combined with the band-limited voice signal, which is usually transmitted through a transmission channel having narrow bandwidth, to produce a combined voice signal having an improved speech quality because of the higher harmonic components added to the band-limited voice signal.

The principle, construction and operations of this invention will be understood from the following detailed discussion taken in conjunction with the accompanying drawings, in which the same and equivalent parts are designated by the same reference numerals, characters and symbols, and in which:

FIG. 1 is a block diagram illustrating an example of this invention;

FIGS. 2 and 3 are circuit diagrams illustrating examples of circuit elements employed in this invention;

FIG. 4 is performance curves illustrative of characteristics of this invention in comparision with those of conventional arts;

FIGS. 5, 6 and 7 are block diagrams each illustrating an actual application of this invention;

FIG. 8 is a block diagram illustrating an example of a circuit used in the example shown in FIG. 7;

FIGS. 9, 10, 11 and 12 are block diagrams each illustrating an example of a higher harmonic generator formed in accordance with this invention; and

FIG. 13 is a block diagram illustrating another example of this invention.

With reference to FIG. 1, an embodiment of this invention comprises an input terminal 1 for receiving a voice signal, switches 2 and 3 used for switching between an ordinary band mode at contacts b and a onehalf band mode at contacts a, a branching hybrid 4 (e.g., a hybrid transformer), an amplifier 5 for amplifying the received signal, a combining hybrid 6, an instantaneous compressor 7, an expander 8, a band-pass filter 9 for obtaining higher frequency components, and a speaker or an earphone 10. The switches 2 and 3 are gang-controlled.

If the voice signal applied to the input terminal 1 has frequency components of a normal frequency band (e.g., 0.3 to 3.4 kilo-Herz), the switches 2 and 3 are switched to contacts b. On the contrary, if the voice signal applied to the input terminal 1 has frequency components of a narrow frequency band (e.g., 0.3 to 1.5 kilo-Herz), the switches 2 and 3 are switched ,to contacts a. In this case, the input voice signal is divided into two parts each having the same waveform as the input voice signal by the branching hybrid 4 as understood from the usual function of a conventional hybrid. One of the hybrid output signals is applied to the amplifier 5, and the other of the hybrid output signals is applied to a cascade-combination of the instantaneous compressor 7 and an expander 8, in which harmonic frequency components of the received voice signal are produced. Necessary higher harmonic frequency components of the voice signal are derived from the bandpass filter 9 and applied to the combining hybrid 6. The amplified voice signal obtained from the amplifier 5 is applied to the combining hybrid 6 with the higher harmonic components obtained from the band-pass filter 9, so that a voice signal approximately the original voice signal is obtained from the combining hybrid 6.

A higher harmonic wave generator comprising a combination of the instantaneous compressor 7 and the expander 8 is described in detail below. It is now assumed that a semi-periodic wave such as a vocalic wave form is an input voice signal which can be expressed by:

1( 2 n'Sin npt z 2 Al Sin MP1 2) If the amplitude component A is less than zero, the value A is given a value IA When the output 2 (1) of the instantaneous compressor 7 is applied to the expander 8 having a square law characteristic, the output e (t) of the expander 8 can be expressed as:

e (t) i (A Sin up!) 3 A component e (I) obtained by eliminating a dc component from the output 2 (1) can be defined as follows:

EU) i ni 211p! M2 iiMz (An 1m) Sin npt Sin mp:

A I Sin 2npt assumed to be a square characteristic. However, the

compression ratio of the compressor 7 may generally be a value l/v. In this case, corresponding expander characteristic is a v-th power characteristic.

With reference to FIG. 2, the instantaneous compressor 7 comprises a pair of transformers T and T connected in cascade, diodes D and D oppositely connected in shunt in a path between the secondary winding of the transformer T and the primary winding of the transformer T With reference to FIG. 3, the expander 8 comprises a transformer T a transformer T and diodes D and D The neutral point of the secondary winding of the transformer T, is connected to the ground. Two terminals of the secondary windings of the transformer T are connected to one electrodes of the diodes D and D The other electrodes of the diodes D and D are commonly connected to one terminal of the primary winding of the transformer T while the other terminal of the primary winding of the transformer T is connected to the ground.

In FIG. 4, input to output characteristics are shown for the abovementioned higher harmonic generator employed in this invention and for an ordinary nonlinear circuit. In this test, a sawtooth wave of 0.5 kilo- Herz is employed as the input signal. Characteristic lines (A), (B), (C) and (D) are higher harmonic characteristics of the second order, the third order, the

- fourth order and fifth order respectively. Dotted characteristic lines (a) and (b) are higher harmonic characteristics of the second order and the third order obtained from the conventional non-linear circuit. As understood from FIG. 4, levels of higher harmonic components generated in accordance with this invention are proportional to the level of the input signal, so that a good proportional relationship between the input to the instantaneous compressor-expander combination and the harmonic output thereof can be obtained in accordance with this invention in comparison with characteristics obtained by a conventional nonlinear circuit.

As mentioned above, a signal substantially approximating an original voice signal can be can be developed from a degraded voice signal in accordance with this invention. In our practical test, a word articulation of about 85 percent was obtained by developing higher harmonics in accordance with this invention, while a word articulation of to percent could be usually obtained by use of an ordinary non-linear circuit as the higher harmonic generator. Moreover, natural quality of the improved voice signal is not less than that of the ordinary telephone service. lri accordance with this invention, a high quality telephone circuit can be realizedby the use of a narrowband transmission signal as mentioned above. Accordingly, this invention can be applied to a receiver for radio telephony using short waves affected by radio interference and noise or to terminal equipment such as a Lincompex system (e.g., Japanese Pat. Publication Nos. 23-9159/1958 and 35-1457/1960) for improving the protection ratio against radio interference or noise and for reducing the degradation of speech quality. The :above merits and effects can be obtained for one-half bandwidth reduction. Accordingly, a transmission channel passband can be divided into two frequency ;bands, through one of which a narrow-band telephone channel can be provided in accordance with this invention, and through the other of which another narrowband telephone channel or a data channel such as a facsimile channel, a telex channel or a stroke signal channel can be provided to provide a complex communicati er ca With reference to FIG. 5, an example of this invention applied to a radio telephone receiver comprises an input terminal 1, a switch 2 for switching between a double-sideband (DSB) mode and the single-sideband (SSB) mode, a DSB intermediate frequency (IF) filter 1 l, a SSB IF filter 12, a DSB detector 13, a SSB detector 14, a branching hybrid 4 (e.g., a hybrid transformer), an amplifier 5, a combining hybrid 6 (e.g., a hybrid transformer), an instantaneous compressor 7, an expander 8, a bandpass filter 9, and a speaker or a headphone 10.

When a received voice signal of normal bandwidth or one-half bandwidth is applied to the input terminal 1, the switch 2 is switched to a contact a or b in accoridance with respective DSB and SSB modes of the received voice signal. If a received wave of DSB mode is applied to the input terminal 1, the frequency band of lthe received wave is limited to one-half thereof by the lDSB IF filter 11 and then applied to the DSB detector '13. A voice signal is detected at the DSB detector 13 and applied to the hybrid 4, from which the output of the DSB IF detector 13 is divided into two parts. One of the two parts is applied to the hybrid 6 through the {amplifier 5. The other of the two parts is applied to a higher harmonic wave generator mentioned above, iwhich is composed of a combination of the instantaineous compressor 7 and an expander 8 for producing lhigher harmonic waves of the received wave. The higher harmonic waves are passed through the bandpass filter 9 and then applied to the hybrid 6 to combine with the output of the amplifier 5 so as to produce an improved voice signal, which is applied to the speaker or headphone 10.

If a received SSB mode signal is applied to the input terminal 1, the frequency band of the received signal is limited to one-half thereof by the SSB IF filter 12 and then applied to the SSB detector 14. A voice signal is provided by the SSB detector 14 and applied to the hybrid 4. Thereafter, an improved voice signal is obtained from the hybrid 6 in a manner similar to the above mentioned DSB mode. As mentioned above, an improved voice signal substantially equivalent to an original voice signal throughout the entire frequency band of the original voice signal is produced. This is useful for reducing effects of radio interference and noise within the transmitted voice signal bandwidth.

With reference to FIG. 6, an example of this invention applied to a radio terminal equipment, such as the above mentioned Lincompex, comprises a receiving antenna 15, a radio receiver 16, a narrow bandpass filter 17, a fading regulator 18, a delay circuit 19 for compensating for the delay difference of the signal in the filter 17 and a bandpass filter 21, a branching hybrid 4, a combining hybrid 6, an instantaneous compressor 7,

an expander 8, an expander 20, the bandpass filter 21,

an amplitude limiter 22, a frequency discriminator 23, an inverse logarithmic circuit 24 and an output terminal 25.

A radio wave including a voice signal and a control signal and having an amplitude fluctuation caused by the transmission medium of normal frequency pass band or one-half frequency pass band is received by the receiving antenna and applied to the receiver 16, whose output is applied to the narrow bandpass filter l7 and the bandpass filter 21. The level of the voice signal is held at a constant level by the fading regulator 18, so that a voice signal of a constant level is applied to the hybrid 4 through the delay circuit 19. The voice signal is divided into two identical signals at the branching hybrid 4. One of the two hybrid output signals is applied to the instantaneous compressor 7, and the other is applied to the combining hybrid 6. The higher harmonic components of the voice signal are produced by the instantaneous compressor 7 and the expander 8 and then applied to the hybrid 6 through the bandpass filter 9. The output of the-hybrid 6 is applied to the expander 20. The other of the two parts (i.e., the control signal) is applied to the amplitude limiter 22 through the bandpass filter 21. The level of the control signal is thus limited to a constant value. The control signal of constant level is applied to the frequency discriminator 23, in which the frequency deviation of the received control signal is converted to an amplitude or level deviation. The output of the frequency discriminator is applied to the expander through the inverse logarithmic circuit 24. The constant level of the above voice signal deviates in accordance with amplitude deviations of the control signal, so that a voice signal substantially equivalent to a voice signal obtained through the entire frequency band is produced from the expander 20.

With reference to FIG. 7, an example of this invention applied to a complex communication system (e.g., a simultaneous transmission system of a telephone signal and a stroke signal through a telephone channel) comprises an input terminal 1, switches 2, 3, 31 and 32 for switching between a simultaneous mode of the telephone and the stroke signal and a single mode of the telephone signal only, a microphone 30, an amplifier 33 for a receiving signal, a branching hybrid 34 for dividing a receiving signal and a sending signal, an amplifier 36 for a received voice signal, a hybrid 37 for combining the received voice signal and a side tone, a receiver 10, a lowpass filter 39 having a cut-off frequency of 1.5 kilo-Herz, an amplifier 40 for a transmitted voice signal, a hybrid 41 for combining a stroke signal and the transmitted voice signal, a hybrid 4 for dividing the stroke signal from a received voice signal, a lowpass filter 35 having a cut-off frequency 1.5, an amplifier 5 for the received voice signal, a hybrid 6 for combining the received voice signal and higher harmonic components from a higher harmonic generator, an instantaneous compressor 7, an expander 8, a bandpass filter 9, a stroke signal transmitter/receiver 42, a bandpass filter 43, an amplifier 44 for a received stroke signal, a stroke signal detector 46, a side tone circuit 38, and a relay 4S employed to switch the switches 2, 3, 31 and 32. The higher harmonic generator is constructed by the instantaneous compressor 7, the expander 8, and the bandpass filter'9.

Since the stroke signal detector 46 resets the relay 45 when the stroke signal is not transmitted, the switches 2, 3, 31 and 32 are switched to contacts a. In this case, the voice signal is transmitted to a transmission line through the microphone 30, the amplifier 33 for the transmitted voice signal, and the hybrid 34. The received voice signal is applied to the receiver 10 through the hybrid 34, the amplifier 36 for the received voice signal, and the hybrid 37.

If the stroke signal is transmitted, the stroke signal detector 46 energizes the relay 45 so that the switches 2, 3, 31 and 32 are switched to the contacts b. In this case, low frequency components of the transmitted voice signal less than the cut-off frequency of L5 kilol-lerz of the lowpass filter 39 pass through the low pass filter 39, the amplifier 40, the hybrid 41, the amplifier 33 and the hybrid 34 and are then applied to the line. The received signal is applied, through the hybrid 34 and the hybrid 4, to the lowpass filter 35. The voice signal is derived from the received signal at the lowpass filter 35 and applied to the instantaneous compressor 7 and the amplifier 5. The output of the amplifier 5 is applied to the hybrid 6. The output of the instantaneous compressor 7 is applied through the expander 8 and the bandpass filter 9 to the hybrid 6. The instantaneous compressor 7 produces higher harmonic components of the voice signal of the frequency band of (0.3 to L5) kilo-Herz, which are applied to the bandpass filter 9. Accordingly, the higher harmonic components of the frequency band of (1.5 to 3.4) kilo-Herz having suitable amplitudes are derived from the bandpass filter 9 and then applied to the hybrid 6. The frequency components of (0.3 to 1.5) kilo-Herz and of (1.5 to 3.4) kilo-Herz are combined at the hybrid 6, so that the combined frequency components of (0.3 to 3.4) kilol-lerz are applied to the receiver 10 through the amplifier 36 and the hybrid 37. The side tone from the sidetone circuit 38 is also applied to the receiver 10.

The stroke signal is produced by the stroke signal transmitter/receiver 42 and applied to the bandpass filter 43. The stroke signal derived from the bandpass filter 43 is applied, through the hybrid 41, the amplifier 33 and the hybrid 34, to the line connected to the terminal 1. The stroke signal from the bandpass filter 43 is also applied to the stroke signal detector 46, so that the relay 45 is energized toswitch the switches 2, 3, 31 and 32 to the contacts b. A received stroke signal is applied, through the input terminal 1, the hybrid 34, the' hybrid 4, the amplifier 44 and the bandpass filter 43, to the stroke signal transmitter/receiver 42.

The switches 2, 3, 31 and 32 may be manually controlled.

With reference to FIG. 8, an example of the stroke signal detector 46 comprises an input terminal 50, an automatic gain controller 51, bandpass filters 52 and 53, rectifiers 54 and 55, lowpass filters 56 and 57, level detectors 58 and 59, and an AND circuit 60 connected to the relay 45.

It is assumed that two frequency-modulated waves f i af and f idf are included in the stroke signal. The stroke signal transmitted and received through the line is applied to the terminal 50. The stroke signal applied to the terminal 50 is then applied to the automatic gain controller 51, in which the level of the stroke signal is regulated to a constant level. The stroke signal with the constant level is then applied to the bandpass filters 52 and 53, which have respectively frequency passbands f i df, and f af Frequency components passed through the bandpass filters 52 and 53 are respectively converted to dc voltages by the rectifiers 54 and 55 and the lowpass filters 56 and 57, each of which has a cutoff frequency of 2 Herz. Each of the level detectors 58 and 59 generates a detected output if the output dc voltage of the lowpass filter 56 or 57 exceeds a threshold level. The detected outputs of the level detectors 58 are applied to the AND circuit 60, whose output energizes the relay 45. By way of example, the output of the AND circuit 60 assumes the state 1 for energizing the relay 45 if the frequency components yf i a f and f, i df continue more than a duration of 500 milliseconds. However, the relay 45 is not energized if the two frequency components f i df and f i df do not continue more than the duration of 500 milli-seconds.

With reference to FIG. 9, another higher harmonic generator comprises an input terminal 101 connected to the above mentioned hybrid 4, an instantaneous compressor 102 corresponding to the instantaneous compressor 7, a phase modulator 103 including a selfoscillator, a frequency converter 104, a bandpass filter.

105, a synchronous oscillator 106, a lowpass filter 107, and an output terminal 108.

it is now assumed that an input signal e, (r) applied to the input terminal 101 is an input composite signal represented as follows:

e,(t) A,l -Sin npt (5) N 620) n -Sin npt However, if the amplitude component A is less than zero, the value A,," is given by a value -IA,, I"? When the output e (r) of the instantaneous compressor 102 is applied to the phase modulator 103 including a selfoscillator of an angular frequency w,,, the output e 0) of the phase modulator 103 can be represented as follows:

e (t) =E Cos (w, t ke (t) This signal e (t) is applied to the frequency converter 104 and the bandpass filter 105. The bandpass filter 103 derives a'component of the center angular frequency w from the signal e (t) indicated by the Equation (7). The component of the center angular frequency w, is applied to the synchronous oscillator 106, so that the oscillation frequency of the synchronous oscillator 106 is synchronized with the center angular frequency W The output e (t) E Cos W t of the synchronous oscillator 106 is applied to the frequency converter 104. Accordingly, the output e (t) of the frequency converter 104 can be indicated as follows:

e (t) E Cos w t Cos (w ke (t) E /2 [Cos ke (t) Cos (2w t ke (t) The lowpass filter 107 derives a frequency component 3 ft from the signal e (t 2tfO1lQWSZ H i) 1 Cos ke (r) N =5, Cos k 2 [4. Sin npt) where A A 8in pt A2 Ag sin 2 A A,," Sin npt The Equation (9) is given by Bessel functions, so that en l s re ses be i tss sfs ls sa Cos kAn=Cos (It/1,3 Sin npt) Sin Mesh (Mn sierra If it is assumed that k" is sufficiently less than l, values Cos A and Sin A can be indicated as follows:

Cos A, z i 14,," /2 (A,,"2/2)2 Cos Znpt l A,,/4 A,,/4 Cos Znpt Sin A, z A,,"2 Sin (2n 1 )pt Accordingly, the Equation (9) can be written as follows:

1 2 2 II' III -Sin npl-Sin mpt As mentioned above, levels of harmonic components obtained at the terminal 108 are proportional to the level of the input signal as clearly understood from the Equation In the example shown in FIG. 9, the instantaneous compressor 102 may be inserted after the phase modulator 103 or the frequency converter 104 for obtaining the same result.

In the example shown in FIG. 9, odd higher harmonic components or even higher harmonic components are obtained. In this case, if the phase position of the output signal of the synchronous oscillator 106 is shifted by a value 90 before application to the frequency converter 104, the mode of the obtained higher harmonic components is changed from the odd components to the even components or from the even components to the odd components. Accordingly, if odd components and even components are necessary, the following example is employed.

In the example shown in FIG. 10, a 90 phase shifter 109, a frequency converter 104a and a lowpass filter 107a are further provided in addition to the example shown in FIG. 9. While the output of the synchronous oscillator 106 is applied to the frequency converter 104, the output of the synchronous oscillator 106 is applied to the frequency converter 104a after phaseshifting of 90 by the 90 phase shifter 109. Accord- (Cos (n m)pt ingly, if the output of the lowpass filter 107 is an odd mode, the output of the lowpass filter 107a is an even mode. Higher harmonic components including an odd mode and even mode are obtained at the output terminal 108.

In the examples shown in FIGS. 9 and 10, the phase modulator 3 including a self oscillator is employed. However, if an external oscillator 110 is employed, the

examples shown in FIGS. 9 and 10 are modified as shown in FIGS. 11 and 12 respectively. Operations in these examples may be readily understood on the analogy of the examples shown in FIGS. 9 and 10, therefore details are omitted.

Input to output characteristics of the higher harmonic generators shown in FIGS. 9 to 12 are similar to the characteristics (A), (B), (C) and (D) shown in FIG. 4.

In the above examples shown in FIGS. 9 to 12, the phase-modulator 103 is employed as the modulator. However, a frequency modulator or an amplitude modulator may be employed in place of the phase modulator 103.

With reference to FIG. 13, the frequency band of the higher harmonic generator may be divided into a pluralit y of frequency bands, so that l evels of the li igher harmonic components can be independently controlled for each of the frequency bands. In FIG. 13, each of the weighting circuits 1, 11 and III (206-1, 206-2, 206-3) is constructed by inductance elements, capacitance elements and/or resistance elements.

A voice signal received from an input terminal 201 (e.g., a voice signal of a limited frequency band (0.3 to 1.5) kilo-Herz) is applied to a hybrid 202 and divided into two parts. One of the two parts is applied to a hybrid 209 through an amplifier 20 8, the other of the two parts is applied to a branching circuit 203. Three outputs of the branching circuit 203 are respectively applied'to filters I, II and Ill (204-1, 204-2, 204-3), which have passbands of (0.3 to 0.7)kilo-I-lerz, (0.7 to 1.1)

Kilo-Herz, and (1.1 to 1.5) kilo-l-lerz respectively by way of example. Three signals separated by the filters I, II and III are respectively applied to higher harmonic generators I, II and 111 (205-1, 205-2, 205-3), from which three parts of higher harmonic components corresponding respectively to the separated three voice signals are obtained. The levels of the three parts of higher harmonic components are independently controlled by the weighting circuits 206-1, 206-2 and 206-3 respectively and combined together at a combiner 207. The higher harmonic components of the input signal of the limited frequency band of (0.3 to 1.5) kilo-Herz are obtained from the combiner 207 and then combined with the output of the amplifier 208 at the hybrid 209 so as to obtain an improved voice signal from an output terminal 210.

Since the levels of the higher harmonic components of the separated frequency bands can be independently controlled in the example shown in FIG. 13, an improved voice signal having natural quality and a desirable character can be obtained.

In the above examples, a voice signal of a limited frequency band of (0.3 to 1.5) kilo-Herz is improved so as to have frequency components of (0.3 to 3.4) kilo- Herz. However, the above frequency bands may be suitably selected in consideration of application purposes. By way of example, a voice signal of an usual frequency band of 0.3 to 3.4) kilo-I-Ierz can be improved so as to have frequency components of (0.3 to 10) kilo- I-Ierz for a broadcast system.

What we claim is:

l. A system for improving the quality of a bandlimited voice signal comprising:

a. a dividing circuit receptive of a band-limited voice signal for providing in response thereto a first and 4 second output signal having waveforms identical to a waveform of said band-limited voice signal;

b. a harmonic signal generator receptive of said dividing circuit first signal for providing in response thereto an output signal comprising harmonics of said band-limited voice signal, said harmonic signal generator comprising an instantaneous compressor circuit receptive of said dividing circuit first signal for providing in response thereto an output signal having frequency components corresponding to frequency components of said band-limited voice signal but reduced in amplitude to 1/v of the amplitude of said band-limited voice signal frequency components where v is a real number, and a level range expanding circuit receptive of said instantaneous compressor circuit output signal for providing in response thereto said harmonic signal generator output signal having high frequency components of a signal equal to the v-th power of said instantaneous compressor circuit output signal; and.

c. a combining circuit receptive of said harmonic signal generator output signal and said dividing circuit second output signal for providing in response thereto an output signal having frequency components corresponding to the frequency components of said band-limited voice signal and harmonics of said band-limited voice signal.

2. A system for improving the quality of a bandlimited voice signal according to claim 1, wherein said level range expanding circuit comprises, an expander having anoutputsignal to input signal ratio equal to v and a band pass filter receptive of the expander output 'signal for passing high harmonic components of the expander output signal.

3. A system for improving the quality of a bandlimited voice signal according to claim 1, wherein said level range expanding circuit comprises, an oscillator providing a carrier signal, a modulator receptive of said carrier signal and said instantaneous compressor circuit output signal for providing an output signal comprising said carrier signal modulated by said instantaneous compressor circuit output signal, and a frequency converting circuit receptive of said carrier signal and said modulator output signal for providing harmonics of said modulator output signal.

'4. A system for improving the quality of a bandlimited voice signal according to claim 2, wherein said frequency converting circuit comprises a frequency converter receptive of said carrier signal and said modulated carrier signal for providing harmonics of said modulator output signal, and a lowpass filter receptive of said harmonics of said modulator output signal for providing high harmonic components from the output of said frequency converter.

5. A system for improving the quality of a bandlimited voice signal according to claim 2, wherein said frequency converting circuit comprises, a bandpass filter receptive of said modulator output signal for extractingsaid carrier signal therefrom, a synchronous oscillator receptive of said carrier signal from said bandpass filter for providing an oscillating output signal synchronized with said carrier signal, a frequency converter receptive of said modulator output signal and said synchronous oscillator output signal for providing harmonics of said modulator output signal, and a low pass filter receptive of said harmonics of said modulator output signal for filtering said harmonics of said modulator output signal.

6. A system for improving the quality of a bandlimited voice signal according to claim 2, wherein said frequency converting circuit-comprises, a bandpass fil- Mtsr r ssptivspfsaisi @9921?! tp t ignal for 9.

tracting said carrier signal therefrom, a synchronous oscillator receptive of said carrier signal from said bandpass filter for providing an oscillating output signal synchronized with said carrier signal, a first frequency converter receptive of said modulator output signal and said synchronous oscillator output signal for providing odd and even harmonics of said modulator output signal, a first lowpass filter receptive of said first frequency converter output signal for providing one of said odd and even harmonics of said modulator output signal, a phase shifter receptive of said synchronous oscillator output signal for providing an output in requency convertor receptive of said modulator output signal and said carrier signal for providing odd and even harmonics of said modulator output signal, a first low pass filter receptive of said firstfrequency con- -'verter output signal for providing one of said odd and even harmonics of said modulator output signal, a 90 ;phase shifter receptive of said carrier signal for providing an output in response thereto, a second frequency converter receptive of said modulator output signal for providing harmonics of said modulator output signal,

and a second lowpass filter receptive of said frequency converter output signal for providing the other of said odd and even harmonics of said modulator output signal.

8. A system for improving the quality of a bandlimited voice signal according to claim 1, wherein said harmonic signal generator comprises; a branching cir-- cuit receptive of said dividing circuit first'output signal for providing a plurality of output signals each identical to said dividing circuit first output signal, a plurality of filters each receptive of one of said branching circuit output signals to provide output signals having different A frequency bands filtered therefrom, a plurality of harmonic generators each receptive of a filter output signal for providing harmonics thereof, a plurality of weighting circuits each receptive of said harmonics of a different one of said filter output signals for providing output signals having frequency components corresponding to said harmonics and amplitudes determined by said weighting circuits, and a second combining circuit for combining said weighting circuit output signals to provide said harmonic signal generator output signal. 

1. A system for improving the quality of a band-limited voice signal comprising: a. a dividing circuit receptive of a band-limited voice signal for providing in response thereto a first and second output signal having waveforms identical to a waveform of said bandlimited voice signal; b. a harmonic signal generator receptive of said dividing circuit first signal for providing in response thereto an output signal comprising harmonics of said band-limited voice signal, said harmonic signal generator comprising an instantaneous compressor circuit receptive of said dividing circuit first signal for providing in response thereto an output signal having frequency components corresponding to frequency components of said band-limited voice signal but reduced in amplitude to 1/v of the amplitude of said bandlimited voice signal frequency components where v is a real number, and a level range expanding circuit receptive of said instantaneous compressor circuit output signal for providing in response thereto said harmonic signal generator output signal having high frequency components of a signal equal to the v-th power of said instantaneous compressor circuit output signal; and c. a combining circuit receptive of said harmonic signal generator output signal and said dividing circuit second output signal for providing in response thereto an output signal having frequency components corresponding to the frequency components of said band-limited voice signal and harmonics of said band-limited voice signal.
 2. A system for improving the quality of a band-limited voice signal according to claim 1, wherein said level range expanding circuit comprises, an expander having an output signal to input signal ratio equal to v and a band pass filter receptive of the expander output signal for passing high harmonic components of the expander output signal.
 3. A system for improving the quality of a band-limited voice signal according to claim 1, wherein said level range expanding circuit comprises, an oscillator providing a carrier signal, a modulator receptive of said carrier signal and said instantaneous compressor circuit output signal for providing an output signal comprising said carrier signal modulated by said instantaneous compressor circuit output signal, and a frequency converting circuit receptive of said carrier signal and said modulator output signal for providing harmonics of said modulator output signal.
 4. A system for improving the quality of a band-limited voice signal according to claim 2, wherein said frequency converting circuit comprises a frequency converter receptive of said carrier signal and said modulated carrier signal for providing harmonics of said modulator output signal, and a lowpass filter receptive of said harmonics of said modulator output signal for providing high harmonic components from the output of said frequency converter.
 5. A system for improving the quality of a band-limited voice signal according to claim 2, wherein said frequency converting circuit comprises, a bandpass filter receptive of said modulator output signal for extracting said carrier signal therefrom, a synchronous oscillator receptive of said carrier signal from said bandpass filter for providing an oscillating output signal synchronized with said carrier signal, a frequency converter receptive of said modulator output signal and said synchronous oscillator output signal for providing harmonics of said modulator output signal, and a low pass filter receptive of said harmonics of said modulator output signaL for filtering said harmonics of said modulator output signal.
 6. A system for improving the quality of a band-limited voice signal according to claim 2, wherein said frequency converting circuit comprises, a bandpass filter receptive of said modulator output signal for extracting said carrier signal therefrom, a synchronous oscillator receptive of said carrier signal from said bandpass filter for providing an oscillating output signal synchronized with said carrier signal, a first frequency converter receptive of said modulator output signal and said synchronous oscillator output signal for providing odd and even harmonics of said modulator output signal, a first lowpass filter receptive of said first frequency converter output signal for providing one of said odd and even harmonics of said modulator output signal, a 90* phase shifter receptive of said synchronous oscillator output signal for providing an output in response thereto, a second frequency converter receptive of said modulator output signal and said 90* phase shifter output signal for providing harmonics of said modulator output signal, and a second lowpass filter receptive of said second frequency converter output signal for providing the other of said odd and even harmonics of said modulator output signal.
 7. A system for improving the quality of a band-limited voice signal according to claim 2, wherein said frequency converting circuit comprises, a first frequency convertor receptive of said modulator output signal and said carrier signal for providing odd and even harmonics of said modulator output signal, a first low pass filter receptive of said first frequency converter output signal for providing one of said odd and even harmonics of said modulator output signal, a 90* phase shifter receptive of said carrier signal for providing an output in response thereto, a second frequency converter receptive of said modulator output signal for providing harmonics of said modulator output signal, and a second lowpass filter receptive of said frequency converter output signal for providing the other of said odd and even harmonics of said modulator output signal.
 8. A system for improving the quality of a band-limited voice signal according to claim 1, wherein said harmonic signal generator comprises; a branching circuit receptive of said dividing circuit first output signal for providing a plurality of output signals each identical to said dividing circuit first output signal, a plurality of filters each receptive of one of said branching circuit output signals to provide output signals having different frequency bands filtered therefrom, a plurality of harmonic generators each receptive of a filter output signal for providing harmonics thereof, a plurality of weighting circuits each receptive of said harmonics of a different one of said filter output signals for providing output signals having frequency components corresponding to said harmonics and amplitudes determined by said weighting circuits, and a second combining circuit for combining said weighting circuit output signals to provide said harmonic signal generator output signal. 